Often wondered about this and I am sure someone here can explain.
How does TCAS deal with an aircraft that is not actually at the FL at which it is reporting to be, due to 1013 not being set at transition?
From my understanding ATC and other aircraft TCAS equipment would observe it to be at the correct FL. The whole time the aircraft could be off the actual FL by the difference between 1013 and QNH which is sometimes quite significant and in extreme cases could be well over 500’ .
I appreciate that TCAS can derive azimuth and distance without any need of altitude input but wondered how it would deal with an apparent conflict where the aircraft were actually vertically separated but appeared to be at the same level and in conflict due to incorrectly set altimeters in one aircraft? Could it give a Descend RA to one aircraft to vacate the apparently common level while in fact descending it towards the other? Or is it "smart" enough to take care of this sort of situation?
I am sure someone can put me straight on this one.